The present invention relates to an electron gun for a cathode ray tube and, more particularly, to an electron gun for improving the resolution of a cathode ray tube by increasing the gap between the main electron beam condensing electrodes of the gun, and thus forming a floating electrode without any applied voltage, in a color cathode ray tube with an in-line-type electron gun.
In the conventional cathode ray tube as shown in FIG. 1, a screen 2 is formed in front of a tube 1, and a shadow mask 3 and a supporting frame 4 for supporting it are installed into the interior of the tube, to the rear of screen 2. An electron gun 5 for projecting an electron beam is installed at the back of the tube 1, and a deflecting yoke 6 is installed in front of the electron gun 5.
If an electron beam is projected from the electron gun 5, the deflecting yoke 6 deflects the electron beam. Then, the electron beam is in collision with the screen 2 by passing through numerous holes formed through the shadow mask 3, so as to form an image.
Thus, the electron gun 5 in the cathode ray tube relies on provision and use of an apparatus for condensing the electron beam projected from its cathode 16 as shown in FIG. 2. This conventional condensing apparatus has first to fourth electrodes 10-13, which are spaced axially apart from one another by respective predetermined gaps and are fixedly supported by bead glasses 14 and 15. As shown in FIG. 3, a pre-focusing lens a is formed by a voltage difference between the second and third electrodes 11 and 12, while an electro-static lens b is formed by a voltage difference between the third and fourth electrodes 12 and 13. Thus, the electron beam passing through the pre-focusing lens a is again condensed on the screen by passing through the electro-static lens b.
However, because the spherical aberration of the electro-static lens b caused by the voltage difference of the third and fourth electrodes 12 and 13 is large, the refractive index of the lens b is varied according to the spherical surface, and thus it is difficult for the electron beam to focus on a point. Also, when the gap between the third and fourth electrodes 12 and 13 is widened to reduce the spherical aberration, an magnetic repulsion effect is generated at the edge of the electro-static lens b by the interference of electric potential induced at the bead glasses 14 and 15, thereby causing deformation of the electro-static lens b. As a result, the convergence of the R(red), G(green), and B(blue) electron beams is varied, depending on time. Thus, the gap between the third and fourth electrodes 12 and 13 can not be widened beyond a predetermined value.
Accordingly, in order to overcome this problem, various methods for increasing the size of the electro-static lens b have been employed. There are two particularly noteworthy methods, of which one enlarges the electro-static lens by enlarging the diameter of the longitudinal central bore of the electrodes, and the other obtains the enlarged hole effect by modifying the composition of the electrical potential of the electrodes. However, in both methods, there is imposed a limit by the sizes of the products, and therefore, a focusing voltage difference between the center and the edge of the electron beam is produced.